This invention relates to a braking system, and more specifically, to a system and method for eliminating fuel use and increasing the maximum braking effort of a particular kind of rail locomotive or off-highway vehicle while in a dynamic braking state.
Diesel electric locomotives generally have two means for slowing a train, where a train consists of at least one locomotive and one rail car, when a need arises. A first means is to apply air brakes that when engaged apply brake shoes to the wheels of a locomotive and/or rail cars to convert the kinetic energy of the train's motion into heat in the wheels, brake shoes and rails. A second means is referred to as “Dynamic Braking” (or DB). In dynamic braking, the electric traction motors of a locomotive are used to slow the train by applying tractive effort to the rail in a direction that produces deceleration of the train. This is accomplished by electrically reconfiguring the traction motors as alternators, when AC traction motors are used, and by dissipating the electrical energy that is produced by rectifying the AC current to DC and heating a large volume of air using electric fans and resistive heating elements. The train is slowed because the energy dissipated is provided by a reduction in the kinetic energy of motion, and thus the speed, of the locomotive and the rail cars. Though it would be highly advantageous to store the energy produced by the traction motors in DB for later use, the high capital cost of the necessary energy storage components has heretofore rendered such a system economically unattractive.
Dynamic braking is used for moderate slowing, in place of the rail car air brakes, and for maximum braking in conjunction with the rail car air brakes. Although dynamic braking functions similar to shoe brakes in converting electric energy into heat, there is little required maintenance with a dynamic braking system other than periodic replacement of the brushes in the DC fan motors. Dynamic braking thus provides cost savings to an operator in contrast with shoe brakes, which inevitably wear and require adjustment and replacement with use.
Another alternative to shoe brakes is used only with vehicles having mechanical transmissions and is known as compression braking or compression retarding. Compression braking is based on rotating the engine with energy supplied by torque from the wheels during deceleration, turning off the fuel, and altering the exhaust valve opening to turn the engine into a compressor. Because the compressed and heated air is discarded to the environment, the engine acts as an energy sink when compression braking is activated and slows the vehicle.
When a locomotive or off highway vehicle with AC traction motors slows in Dynamic Braking, the engine continues to consume fuel to meet the electrical needs of the vehicle. On an AC locomotive, for example, this includes power for the traction motor blowers, radiators fan(s), air compressor, operator cab heater or air conditioner, alternator blower, eductor blower, and battery charging circuit, among others.
There are conditions where the rotational speed of a vehicle's wheels differs from that associated with ‘rolling contact’ between the wheel and the driving surface. In common parlance, if the periphery of a wheel moves faster than the speed of the vehicle, as during hard acceleration, the condition is known as wheel-spin or wheel-slip, and if the periphery of the wheel moves slower than the speed of the vehicle, as during hard braking, the condition is known as wheel-slide, or in the extreme, locked wheels. In either case, the condition should be avoided because damage can be done to the wheels and/or driving surface when these occur. Moreover, when it is imperative that a rapid vehicle stop be made, as for example when approaching a bridge that has failed, the maximum deceleration will occur only if the wheel(s) periphery speed is close to the speed of the vehicle. In other words, if the rotation of the wheel slows or stops and sliding ensues, the vehicle will take longer to stop. Accordingly, locomotive, off-highway, and some road vehicles contain anti-slip and anti-slide control which compares the rotational speed of the wheels against a measured or calculated vehicle speed and which actively reduces the torque to the wheels momentarily to re-establish rolling contact between the wheels and the driving surface. The control then reapplies torque to achieve the desired acceleration or deceleration of the vehicle without allowing wheel slip or wheel-slide. Under conditions of maximum acceleration or deceleration on slippery surfaces, the anti-slip or anti-slide control may vary the torque at the wheels many times a second to preserve rolling contact and provide maximum performance.
Manufacturers and operators of locomotives and off highway vehicles would benefit from a system which would eliminate or greatly reduce the use of fuel when in a dynamic braking mode where power is still needed to operate an engine, alternator and/or auxiliary alternator.